Redox-flow batteries (RFB) are attracting broad interest for electrical energy storage in solar and wind power systems and large-scale electric grids because of their low-cost, safety and small environmental footprints. The RFB operates on electrode reactions of dissolved red-ox metal ion couples separated by an ion exchange membrane (IEM). The IEM is ideally electronically insulating and highly permeable to the nonreactive ion charge carriers but impermeable to the reactive metal ions. To date, RFBs are mostly based on proton-selective, perfluorinated or non-perfluorinated ionic polymer IEMs. However, in the extremely acidic and oxidizing RFB electrolyte solutions, the polymeric IEMs have common issues of metal ion crossover and material degradation over long term that limit the cell efficiency and the lifetime.
Zeolites are crystalline aluminosilicates with enormous internal surface area, large porosity and uniform pore diameters ranging from 0.3 nm to over 1 nm depending on the specific crystallographic structure. The zeolite framework is formed by [SiO4] and [AlO4] tetrahedrons interlinked through corner oxygen ions and large numbers of exchangeable extraframework cations exist in the zeolitic channels as charge compensators for [AlO4] sites. The pore size and chemical and physical properties of zeolite materials can be fine-tuned by framework isomorphous elemental substitution and extraframework ion exchange during and after synthesis. Over the last few decades, various types of zeolite membranes have been developed for gas and liquid separations based on molecular size discrimination or competitive molecular adsorption-diffusion mechanisms. In recent years, zeolite membranes were also demonstrated for water purification from salt solutions by size-exclusion (steric) effect because metal ions are bonded with surrounding water molecules to form hydration shells making the hydrated ion size too large to enter the zeolitic pores. The kinetic size of hydrated metal ion increases with the ion charge density, i.e. electrical charge per volume of the ion. Unlike metal ions, protons in aqueous solutions exists in the form of H3O+ (hydronium) with three identical “H—O” bonds making it a polyatomic ion with charge density too small to form a definable hydration shell. The kinetic size of the H3O+ is thus much smaller than the hydrated multivalent metal cations commonly involved in various RFB systems.